Fullerenes are carbon cages containing a central cavity. These molecules with the composition C.sub.20+2m, where m is an integer, can take the stable form of hollow closed nets composed of pentagons and hexagons. The discovery of Buckminsterfullerene, a C.sub.60 spherical allotrope of carbon, in 1985 by Kroto, H. W., Heath, J. R., O'Brien, S. C., Carl, R. F., Smalley, R. E.; "C.sub.60 : Buckminsterfullerene"; Nature, Vol. 318, November 1985, pp. 162-163 has precipitated a flurry of activity directed towards understanding the nature and properties of fullerenes, particularly their use as lubricants, semiconductors and superconductors. This research has been significantly hampered by the difficulty in obtaining gram or larger quantities of pure materials.
To date, fullerenes have been synthesized using a laser to ablate graphite, burning graphite in a furnace or by producing an arc across two graphite electrodes in an inert atmosphere. By impregnating graphite with metal salts or oxides, or conducting the vaporization in a metal containing atmosphere, a metal encapsulated in a fullerene can be synthesized. Other techniques applied to synthesize fullerenes include negative ion/desorption chemical ionization and a benzene flame. In each case, a soot comprising a mixture of C.sub.60 and C.sub.70 fullerenes, and even higher numbered carbon molecules is obtained. For example, carbon arc soot contains about 80-85% C.sub.60, 10-15% C.sub.70, and 5% higher fullerenes.
The first and still most commonly used method for purifying crude C.sub.60 fullerene is by column chromatography on activity grade I neutral alumina using 5% toluene in hexane as the eluent. However, C.sub.60 fullerene is only very slightly soluble in toluene/hexane (5/95) and this low solubility requires the use of large quantities of solvent and very large columns. The use of larger fractions of toluene in hexane afford no separation of C.sub.60 from the higher fullerenes. Using this method to purify 500 mg of crude fullerenes requires large quantities of materials; 2500 g of alumina and about 12 liters of solvent making the process relatively expensive. Another disadvantage of this method is that alumina having a high activity, i.e., grade I alumina, tends to irreversibly adsorb C.sub.60. During a typical separation procedure, only about 50% out of a possible 80% of C.sub.60 present in the crude can be recovered. Purification using this method can take as long as 8-12 hours due to the large size of the columns necessary to purify 500 mg of crude fullerenes.
In another chromatographic method, powdered graphite has been used as the stationary phase. Vassallo, A. M.; Palisano, A. J.; Pang, L. S. K., Wilson, M. A.; "Improved Separation of Fullerene -60 and -70"; J. Chem. Soc., Chem. Comm., 1, pp. 60-61 (1992). Higher toluene concentrations (10% toluene in hexane) make it possible to use less solvent. However this method yields a poor recovery of C.sub.60, typically giving only 32% pure C.sub.60 as compared to a possible 80% yield.
Gel permeation chromatography (GPC) has also been used for C.sub.60 purification. Meier, M. S., Selegue, J. P.; "Efficient Preparative Separation of C.sub.60 and C.sub.70 Gel Permeation Chromatography of Fullerenes Using 100% Toluene as Mobile Phase"; J. Org. Chem., 57, pp. 1924-1926 (1992). In this technique 100% toluene is the eluent. Since C.sub.60 is more soluble in toluene than in toluene hexane mixtures, this technique has the advantage of requiring smaller solvent volumes. This method gives 50% recovery of C.sub.60 out of a possible 80%, with the remaining C.sub.60 eluting as an impure fraction that requires multiple re-injection and purification steps. The major disadvantages of this method are the need for an high pressure liquid chromatography (HPLC)/GPC apparatus, the tremendous cost of the separation columns, and the inapplicability for near gram scale separations.
Finally, a method of chromatographic purification of C.sub.60 using multi-legged phenyl groups bound to silica gel as a stationary phase has been reported. Jinno, K., Kunihiko, Y., Takanori U., Hideo N., Kenji, I.; "Liquid Chromatographic Separation of All-Carbon Molecules C.sub.60 and C.sub.70 With Multi-Legged Group Bonded Silica Phases"; J. Chromatogr., 594, pp. 105-109 (1992). This method necessitates the utilization of custom-made stationary phases and has only been demonstrated for analytical scale separations, not for preparative purposes.